Islamic State’s taunt that “we love death more than you love life” was always a threat as well as a fact. This expressed love of death was incomprehensible to people raised to believe that self-preservation is at the core of human nature. In extremis, soldiers might die to protect their comrades and parents would sacrifice their own lives to save their children. But they did not love death, let alone long for it.

A love of death meant ISIS was implacable. There was no possible negotiation, and no deterring suicide bombers or terrorist attacks in which militants knew they would die. In fact, getting themselves killed was part of their motive. It didn’t make sense.

But it makes perfect sense to violent jihadists bent on heroic deeds. It’s at the core of ISIS ideology, and also that of other jihadists such as al-Qaeda’s 9/11 hijackers.

Death, in the minds of ISIS fighters, is not the end of life. It’s the way to an eternal life of bliss in Heaven, to a Paradise depicted in the most extravagant, sensual terms in certain Muslim writings that are accepted as literal truth. The Quran itself has many, and many more are found in the Hadith, which are later commentaries on the Quran that claim to report, verbatim, what Prophet Muhammad said on various matters.

For an Islamist fundamentalist, Heaven is a real place, a Garden full of sensual delights. For example, take the famous claim that in Heaven a deserving Muslim will enjoy the company of 72 virgins: This is nowhere mentioned in the Koran. It is found in a fanciful hadith by Ibn Kathir, writing in the 14th century, hundred of years after Mohammed’s death: “The Prophet Mohammed was heard saying, ‘The smallest reward for the people of paradise (meaning of course only men) is an abode where are 80,000 servants and 72 wives, over which stands a dome decorated with pearls, aquamarine, and ruby…’” Innumerable such tall tales appear throughout the history of Islamic commentary.

In ISIS ideology, Heaven is not a diluted doctrine as it has become for many Christians. Heaven in jihadi belief is an operational concept. To die as a martyr in battle or a suicide bombing means, literally, immediate ascent to Heaven and the beginning of life in Paradise.

The idea of Heaven, the vision of Paradise, is ISIS’s secret weapon, the source of its willingness to fight to the end. Convincing fighters that Paradise is real, that it’s a certainty because they die as martyrs to jihad, is what made Islamic State’s military forces so fearsome, its suicide bombers so ecstatic. In recent battles, wounded Islamic State fighters defending impossible positions have mocked attacking coalition forces as apostates and infidels, demanding to be killed. Blowing themselves up rather than be taken prisoner was always an ISIS commandment.

The trick in ISIS ideology was to persuade militants that martyrdom meant that ‘they would get to Heaven faster.’ Similar to this was the explanation that barbaric violence in newly conquered cities and towns — the beheadings, crucifixions, the burning of captives in cages, or the drowning of women who refused to become sex slaves — is justified because ‘more violent methods now means fewer injuries later.’ For some fighters it’s just bloodlust. But the depth of ideological conviction in the organization as a whole cannot be underestimated. ISIS has never been just a gang of criminals and murderers.

Kamel Daoud, an Algerian writer, says that “[t]oday, one has to be a Muslim — by faith, culture or place of residence — in order to experience the full weight” of the idea of Heaven in the psychology of the popular imagination, especially of a desperate younger generation. “Paradise has come back into fashion, described in mind-boggling detail by preachers, imams, and Islamist fantasy literature.” The main selling point is women. The vision of “the women of paradise … feeds a barely believable form of erotico-Islamism that drives jihadists and gets other men to fantasize about escaping the sexual misery of everyday life. Suicide bombers or misogynists, they share the same dream.” To understand ISIS, you have to understand its psychology.

On July 2, jihadi militants in Dhaka, Bangladesh invaded the Holy Artisan Bakery, killing 20 foreigners then mutilating their bodies with machetes. Bangladeshis, all Sunni Muslims, were spared. The attackers were polite and solicitous to the Muslim Bengali staff, asking that coffee and tea be served and that dishes of fish and shrimp be prepared. The killers were sons of the Bangladeshi elite — well-educated, handsome — they spoke cosmopolitan Bengali and English, having been to English-speaking schools. One of the staff said later, “If you look at those guys, nobody could believe they could do this.”

Toward the end, one of the killers said to the Bengalis, “You see what we did here … The same thing is going to happen to us now.”

At the end of an overnight standoff, at 7:30 a.m. he told them, “We are leaving. See you in Heaven.” There can be little doubt he meant this literally. Just as they prepared to exit the restaurant, guns blazing, the commando raid was launched. They were of course killed, as they wished.

The trouble with the idea of Heaven is that for true believers it can justify creating Hell on earth. In a nutshell, that’s the story of ISIS.